1. Field of the Invention
The present invention relates to a substrate for an optical recording medium, particularly suitable for a magneto-optical recording medium capable of direct overwrite by modification of power level and/or pulse width of the recording optical pulse, and also relates to such a magneto-optical recording medium using said substrate.
2. Description of the Related Art
Optical discs have been intensively investigated, developed and commercialized as high density and capacity information storage media. The commercialized optical discs have, adjacent to data areas, a guide in the form of a convex or concave portion on the surface of the substrate for servo tracking by an optical beam during recording, reproduction, erasing, etc., of information. The typical guide is a groove formed on the surface of the substrate in the form of spiral or concentric circles. The control of the tracking servo is conducted by a light reflected from the guide.
It is however known that the quality of signal is degraded by the guides existing in the vicinity of the data areas. The convex or concave guides reflect to the recording layer which causes a deformation of bit configuration lowering the C/N ratio (carrier to noise ratio), etc.
Some solutions have been proposed to solve the above problem. For example, U.S. Pat. No. 5,089,358, issued on Feb. 18, 1992 for Taki et al, discloses a flat layer formed on a base plate to provide a flat surface on which a recording layer is formed so that the signal reproduced from the recording layer is not deteriorated. Taki et al form the guides by patterning a metal reflecting layer deposited on a glass base plate, followed by making the flat layer on the guides. This process requires complex steps such as photolithography and etching and is not adequate for mass production due to low yield and high cost, etc.
JP-A-57-60544, published on Apr. 12, 1982, discloses a leveling resin layer formed on a resin substrate having convex or concave portions, as a tracking servo guide on the surface thereof. In JP-A-57-60544, since both the planalizing layer and the substrate are made of a resin, the light reflection from the tracking servo guide is disadvantageously low due to a small difference of the refractive index thereof so that the tracking servo is not efficient.
JP-A-2-152041, published on Jun. 12, 1990, discloses a leveling layer of silicon nitride on a substrate by sputtering or evaporating silicon nitride onto a substrate while milling the surface of the deposited silicon nitride layer. This sputtering or evaporation while milling is complex and does not provide a planalizing layer having a desired planer top surface.
JP-B2-4-47910, published on Aug. 5, 1992, discloses a thin film coating on a substrate having convex or concave portions as tracking servo guides, in which the thin film coating covers the sharp angle edges of the convex or concave portions and provides a relatively flat or smooth top surface by which a recording layer to be formed thereon would not be damaged. In JP-B2-4-47910, if the thin film coating is an organic resin layer, a sufficient difference in the refractive index between the thin film coating and the substrate cannot be obtained, and if the thin film coating is an inorganic layer, the top surface of the thin film coating cannot be made geometrically planar, causing the S/N ratio to be lowered due to still existing geometrical convex or concave portions.
The object of the instant invention is to solve the above problems, to provide a substrate for an optical recording medium by which a reliably high tracking signal is obtained from the guides for a tracking servo and an enhanced quality of reproduced signal can be obtained due to a recording layer being formed on a geometrically planar surface of the substrate, and which can be produced at a high yield and a low cost, and to provide a magneto-optical recording medium using such a substrate.